Action of Alkalis.—­Alkalis have a very considerable action on fur and wool, but the effects vary a good deal according to the kind of alkali used, the strength and the temperature of the solution, as also, of course, the length of period of contact.  The caustic alkalis, potash and soda, under all conditions affect wool and fur injuriously.  In fact, we have a method of recovering indigo from indigo-dyed woollen rags, based on the solubility of the wool in hot caustic soda.  The wool dissolves, and the indigo, being insoluble, remains, and can be recovered.  Alkaline carbonates and soap in solution have little or no injurious action if not too strong, and if the temperature be not over 50 deg.  C. (106 deg.  F.).  Soap and carbonate of ammonium have the least injurious action.  Every washer or scourer of wool, when he uses soaps, should first ascertain if they are free from excess of alkali, i.e. that they contain no free alkali; and when he uses soda ash (sodium carbonate), that it contains no caustic alkali.  Lime, in water or otherwise, acts injuriously, rendering the fibre brittle.

Reactions and tests proving chemical differences and illustrating modes of discriminating and separating vegetable fibres, silk and wool, fur, etc.—­You will remember I stated that the vegetable fibre differs chemically from those of silk, and silk from wool, fur, and hair, in that with the first we have as constituents only carbon, hydrogen, and oxygen; in silk we have carbon, hydrogen, oxygen, and nitrogen; whilst in wool, fur, and hair we have carbon, hydrogen, oxygen, nitrogen, and sulphur.  I have already shown you that if we can liberate by any means ammonia from a substance, we have practically proved the presence of nitrogen in that substance, for ammonia is a nitrogen compound.  As regards sulphur and its compounds, that ill-smelling gas, sulphuretted hydrogen, which occurs in rotten eggs, in organic effluvia from cesspools and the like, and which in the case of bad eggs, and to some extent with good eggs, turns the silver spoons black, and in the case of white lead paints turns these brown or black, I can show you some still more convincing proofs that sulphur is contained in wool, fur, and hair, and not in silk nor in vegetable fibres.  First, I will heat strongly some cotton with a little soda-lime in a tube, and hold a piece of moistened red litmus paper over the mouth of the tube.  If nitrogen is present it will take up hydrogen in the decomposition ensuing, and escape as ammonia, which will turn the red litmus paper blue.  With the cotton, however, no ammonia escapes, no turning of the piece of red litmus paper blue is observed, and so no nitrogen can be present in the cotton fibre.  Secondly, I will similarly treat some silk.  Ammonia escapes, turns the red litmus paper blue, possesses the smell like hartshorn, and produces, with hydrochloric acid on the stopper of a bottle, dense white fumes of sal-ammoniac (ammonium chloride).  Hence silk